This invention relates to a method of purifying molten chloride salts used in the reprocessing of Integral Fast Reactor (IFR) fuel. More specifically the invention relates to a process of removing rare earths fission products from mixed chloride salts used in the electrolytic reprocessing of irradiated IFR fuels.
The Integral Fast Reactor (IFR) is a sodium-cooled, fast reactor using metallic alloys of uranium, plutonium and zirconium as fuel, and having a close-coupled fuel cycle. Close-coupling of the reactor and the fuel cycle facilities can be achieved by locating on one site the reactor and the facilities for reprocessing spent fuel, fabricating recovered fuel, and managing fission product wastes. With this arrangement, it is not necessary to ship fuel to or from the reactor site. Alternatively, spent fuel from several reactors could be treated at a central facility. In either case, reducing the volume of waste product produced from fuel reprocessing is inherent in and required by the IFR concept.
The recovery of fissionable materials such as uranium and plutonium from spent nuclear reactor fuels can be carried out by electrorefining methods using electrochemical cells of the type described in U.S. Pat. Nos. 4,596,647 and 2,951,793, as well as U.S. Pat. No. 4,880,506. It is the electrorefining method which is being developed for the reprocessing of IFR fuel. In a typical electrorefining cell an electrolyte consisting of a molten chloride salt mixture, such as the eutectic mixture of LiCl and KCl, is used to transport the metal or metals to be purified from anodes to cathodes. When used to reprocess spent nuclear reactor fuels, the salt mixture eventually becomes contaminated with radioactive rare earth metals which causes excessive heat built-up and contamination of the uranium or plutonium product salt such that eventually the salt is no longer suitable for use in the electrorefining cell.
Previously, the design for salt purification and removal of rare earths was based on a series of individual batch operations utilizing extracting agents such as Cd-U solutions and reducing agents such as Li-Cd alloys or Li-K-Cd alloys in order to recover actinides and isolate the rare earths in a metal waste matrix, all as previously disclosed in the Johnson et al. U.S. Pat. No. 4,814,046. This process required that the electrorefining process be shut down. This increased considerably the overall time to reprocess spent fuel and required use of the electrorefiner for certain of the operations.
A new process has been developed which operates external to the electrorefiner to rapidly treat at least a portion of the electrolyte salt in a series of steps to remove the rare earth contaminants from the salt while leaving the salt in such a condition that it can be recycled directly back into the electrorefiner with the recovered actinides.